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RM0312 Datasheet(PDF) 236 Page - STMicroelectronics |
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RM0312 Datasheet(HTML) 236 Page - STMicroelectronics |
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236 / 275 page  Universal synchronous/asynchronous receiver transmitter (USART) RM0312 236/275 DocID022352 Rev 3 Note: The lower the fMASTER frequency, the lower the accuracy for a particular baud rate.The upper limit of the achievable baud rate can be fixed with the above data. 21.3.5 USART receiver’s tolerance to clock deviation The USART’s asynchronous receiver works correctly only if the total clock system deviation is smaller than the USART receiver’s tolerance. The causes which contribute to the total deviation are: • DTRA: Deviation due to the transmitter error (which also includes the deviation of the transmitter’s local oscillator) • DQUANT: Error due to the baud rate quantization of the receiver • DREC: Deviation of the receiver’s local oscillator • DTCL: Deviation due to the transmission line (generally due to the transceivers which can introduce an asymmetry between the low-to-high transition timing and the high-to- low transition timing) DTRA + DQUANT + DREC + DTCL < USART receiver’s tolerance The USART receiver’s tolerance to properly receive data is equal to the maximum tolerated deviation and depends on the following choices: • 10- or 11-bit character length defined by the M bit in the USART_CR1 register • Use of fractional baud rate or not Table 46. Baud rate programming and error calculation Baud rate fMASTER = 16 MHz In bps Actual % Error(1) 1. Error % = (Calculated - Desired) Baud Rate / Desired Baud Rate USART_DIV BRR1 BRR2 2.4 2.4 0.0 2710h 71h 20h 9.6 9.6 0.0 09C4h 9Ch 04h 19.2 19.2 0.0 04E2 4Eh 02h 57.6 57.554 -0.08 01A1h 1Ah 01h 115.2 115.385 0.16 00D0h 0Dh 00h 230.4 230.769 0.16 0068h 06h 08h 460.8 461.538 0.16 0034h 03h 04h 921.6 923.077 0.16 001Ah 01h 0Ah Table 47. USART receiver’s tolerance when USART_DIV[3:0] is 0 M bit NF is an error NF is don’t care 0 3.75% 4.375% 1 3.41% 3.97% |
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